Fastener assemblies are simple machines but they are often used in very complex structures. When working in situations where joint failure is catastrophic making assumptions about how well they are working is not an acceptable method of operation. In these cases liability and refastening costs are so high that it is essential to understand the hows and whys of fastening with bolts and nuts.

It's all About Clamping Force

As you turn the nut onto the bolt, it causes the threads to press tightly against each other. As tightening continues, the desired clamping force is achieved — the nut pulling in one direction, the bolt resisting from the other.Naturally, when the bolt and nut are this tight, resistance to further turning is very high. Your torque wrench will tell you this but it is not “torque” that determines bolt and nut tightness. It is "tension."

Tension

Tension is an indication that the length of the bolt shaft is being stretched. . . actually getting longer. You can’t see it with the naked eye, but that’s what’s happening.

Torque

If Nuts Could Talk

Torque wrenches provide only part of the information you need about fastener assemblies. They tell you how much resistance they are getting from further turning of the nut. What does that resistance mean? Is the nut saying "Okay, every one of my threads is in tight contact with those on the bolt and I'm creating a tight, secure joint." Or is it saying "The rust on this bolt is catching on my threads making it hard to turn.” The dial indications may be the same for both situations so a fixed amount of torque may not always produce the same tension.

Here are some other variables that can affect torque:

Hardness and strength of the fasteners

Lubricants

Types of washers

Thread condition

Surface characteristics of the parts being assembled (plating, coating, hardness or smoothness)

Improperly maintained or underpowered power tools

Operator “feel”

Because our Skidmore-Wilhelm units test tension directly by placing the actual fastener assembly in the tester you can see the affect all the joint variables have on the amount of torque needed to produce the required tension. Using a Skidmore bolt tester in conjunction with torque wrench provides an exact torque-tension relationship.